This invention relates generally to systems capable of determining the spatial position and angular orientation (i.e. pose) of three-dimensional (3D) bodies or objects. More specifically, the invention relates to systems capable of tracking objects in real time within a specified volume, without regard to the objects' rigidity or visibility.
As is known in the art, a wide variety of systems have been developed that can determine the spatial position and angular orientation of objects over small time-scales, and thus track their motion in real time. These systems generally make use of specific physical phenomena, and as such, have different capabilities and limitations. One system is an optical system. Such an optical system operates on sensing of sources of radiated electromagnetic energy (e.g., light or infrared energy emitted from active markers or light or infrared energy reflected from passive markers) by sensitive arrays, such as charge coupled devices (CCD). Such optical systems can provide highly accurate spatial and angular measurements at high sampling frequencies over large operational volumes (typically room size), but require that a minimum number of the markers always be in view of the CCD sensors. This line-of-sight limitation can be partially remedied by determining the position of an obscured point from the measured positions of the visible markers with triangulation techniques. For example, markers can be affixed to instruments or probes (such as surgical probes) such that their tip points can be tracked. However, such probes must be rigid. This method cannot be applied to flexible probes such as catheters.
Another type of system is a non-optical system. Such systems include magnetic systems, mechanical systems, and ultra-sonic systems. For example, U.S. Pat. Nos. 5,197,476 and 5,295,483 to Nowacki and Horbal disclose the use of optical cameras to track the position of an ultrasonic scanner or probe, which itself detects concretions such as kidney stones within the human body. The ultrasonic scanner cannot determine the tracked object's pose, though. Magnetic systems do not suffer the line-of-sight problem inherent in optical systems; but, such systems can be severely affected by extraneous objects perturbing their magnetic fields, and are also generally less accurate. Mechanical systems use mechanical devices, such as articulating arms, and are free of line-of-sight and magnetic disturbance problems; but, such systems are considerably more expensive for a given level of accuracy. Their accuracies are subject to perturbations arising from gravitationally induced forces and torques, which limit them to the generally smaller operational volumes spanned by their range of motion. Also, they are more cumbersome than other devices since their motion is constrained by possible collisions with other objects lying within their operational volume.
As is also known in the art, optical devices can be used in conjunction with non-optical devices to overcome the optical devices' line-of-sight-limitations, but such coupling inconveniently results in the measured position data being reported in separate frames of reference, thus requiring the data to be reconciled by the user calibrating the devices to determine the necessary transformation between the two frames of reference. Birkfellner, et. al., Concepts and Results in the Development of a Hybrid Tracking System for CAS, Lecture Notes in Computer Science: Medical Image Computing and Computer-Assisted Intervention--MICCAI'98, Vol. 1496 (1998), pp. 342-351, describe such a system comprised of an optical tracking system and a direct current pulsed electromagnetic tracking system. They also describe procedures for calibrating and registering the magnetic system local frame of reference to the optical system frame of reference. Their system requires that the magnetic field source remain fixed after the lengthy calibration and registration procedures have been done, thus discouraging the movement of the field source to other convenient or appropriate positions as may be desired. Also, because their system reports position data from the magnetic subsystem only when the optical position data is unavailable because of obstructions in the optical system's line-of-sight, it remains essentially an optical system that is augmented by a non-optical system.
U.S. Pat. No. 5,831,260 to Hansen teaches a hybrid motion tracker having magnetic and optical sub-systems. This system is used for motion capture, using sensor assemblies (having both magnetic field sensors and optical Light Emitting diode (LED) sources) placed strategically on the person(s) being tracked to detect the motion. In normal operation, the optical sub-system provides the 3D position data, because of its inherently greater accuracy, and the magnetic sub-system provides the orientation data; if the optical sources are obscured, then the magnetic sub-system also provides the position data. Hansen's system is similar to the system described by Birkfellner, et. al, being primarily a coupling of a commercially available magnetic sub-system with a commercially available optical system, and using commercially available software to transform measurements between the subsystems (although some integration exists, such as using the optical sub-system to compensate the magnetic sub-system's degrading Signal-to-Noise ratio). Thus Hansen's system suffers the same deficiencies inherent in such systems, requiring fixed magnetic transmitters and fixed optical sensors whose fixed frames of reference must be first registered by the user by means of lengthy calibration and registration procedures, thereby precluding any easy repositioning of the sub-systems relative to one another as conveniently desired.